With the rapid advancement of advanced information-oriented society in recent years, development of information processing apparatus such as computers and communication apparatus, and image processing apparatus such as camcoders and digital cameras is being rapidly advanced. In the field of these electronic apparatus, there is a strong demand for the enhancement of throughput capacity, i.e., the speeding up, and miniaturization and weight saving thereof. In this field, mounting parts such as semiconductor chips such as LSI, circuit parts and functional parts are mounted on a wiring board (substrate), and this wiring board is incorporated into an electronic apparatus. It is effective for the speeding up of information processing, image processing and the like to make the interconnected wiring of these mounting parts as short as possible so as to make the density of the parts high. This technique is also effective for the miniaturization and weight saving of electronic apparatus.
The use of a flexible substrate made of a synthetic resin film as a wiring board itself is effective to realize high-density assembly and the miniaturization and weight saving of electronic apparatus. Therefore, thin and freely flexible substrates are used in circuit boards such as printed wiring boards.
Even in a mounting system for electronic parts such as semiconductor chips, a system that semiconductor chips are mounted on a wiring board as bare chips is being developed in place of the conventional mounting system by transfer molding. In this mounting system, for example, TAB (tape automated bonding) in which a package itself for LSI is a plastic film (including a sheet) is known. TAB is generally formed from a tape-like carrier film, bumps (metallic projections) and a semiconductor chip. In such a mounting system, a flexible synthetic resin film having a wiring pattern is often used as the carrier film.
Polyimide has heretofore been used as a material for substrates of flexible printed wiring boards and films having a wiring pattern. However, a polyimide film is excellent in mechanical properties and flame retardant, but involves a problem that water absorptivity is high. Therefore, the use of the polyimide film as a substrate for high-density assembly has involved a problem that reliability on insulation is lowered due to water absorption, and the dielectric constant of the substrate is lowered due to moisture absorption. Accordingly, the polyimide film cannot be said to be a substrate material sufficiently satisfying recent high requirements.
On the other hand, ring structure-containing polymers such as ring-opening polymers and addition polymers of norbornene monomers such as tetracyclododecene are publicly known as insulating materials excellent in low water absorption property and dielectric properties, and techniques that glass cloth and the like are impregnated with these polymers to use them as substrates for printed wiring boards and that they are used in the form of a sheet as insulating films have been proposed (for example, Japanese Patent Application Laid-Open No. 248164/1994). However, these ring structure-containing polymers are generally insufficient in mechanical strength and flexibility and also poor in flame retardant, and so it has been difficult to use them singly as wiring boards and wiring films.
As described above, there has not been yet found any synthetic resin film which has sufficient mechanical strength and flame retardant as a film for flexible wiring boards and is excellent in low water absorption property and dielectric properties. There is a demand for development of novel films highly balanced among these various properties as materials for high-density assembly for realizing the speeding up of processing and the miniaturization and weight saving of apparatus.